The present invention relates to the field of cementing pipe in wellbores. More particularly, the present invention relates to an improved apparatus and method for improving the bond between the downhole pipe and cement place in the wellbore.
The production of oil and gas requires production casing to be set in the wellbore. The production casing provides a conduit for transporting the hydrocarbon fluids from the subsurface formation to the well surface. To prevent the migration of the hydrocarbon fluids around the exterior of the casing, and to prevent the dilution of the hydrocarbon fluids with water produced from other strata, cement is placed in the annulus between the exterior of the casing and the interior of the wellbore.
To facilitate the formation of a strong cement bond around the casing, pipe centralizers typically are installed on casing sections to center the casing in the wellbore so that an even thickness of cement is located on all sides of the casing. This technique restricts channeling of the cement, and the creating of voids in the cement, caused when the casing rests against the wellbore surface.
Several factors adversely affect the cementing process. Mud cake on the wellbore surface can prevent a good bond between the cement and the wellbore. To avoid this problem, scratching tools having wires or bristles are run in the wellbore to dislodge mud cake from the wellbore. Additionally, the dehydration of the cement as the cement cures causes shrinkage of the cement, and the possibility of hairline cracks in the bond between the casing and cement. This differential contraction is heightened by temperature variations in the wellbore, casing, and cement. While wellbore temperatures may exceed 350 degrees F, the initial temperature of the casing may be below freezing in winter operations. Similarly, the temperature of the cement is typically different than the wellbore and casing temperatures. All of these potential variations in temperature can adversely affect the ability of the cement to form a solid seal between the casing and the wellbore.
The bond between the casing and the cement is also restricted by the formation of mud cake on the exterior surface of the casing. When the casing is run into the wellbore, the casing contacts drilling mud and forms a hard cake or impurities on the exterior surface of the casing. This mud cake can be extremely hard when subject to elevated temperatures in the wellbore. While floats such as rubber plugs are typically run through the interior of the casing during cementing operations to swab the interior of the casing, no steps are taken to clean the outside surface of the casing before the cement is placed in the wellbore. The failure to remove the mud cake from the exterior surface of the casing can leave a microannulus leading to failure of the cement seal.
In deep wells, the bottom hole pressure can exceed 18,000 psi. If a hairline fracture or microannulus permits the migration of fluids from a water zone to a hydrocarbon producing zone, the water will be forced through such fracture at high pressures. This migration will cause erosion of the cement or softer mud cake, and can eventually open a large channel between the water and hydrocarbon producing zones. If enough water dilutes the hydrocarbon producing zone, additional expense is incurred to produce the hydrocarbon fluids, and total hydrocarbon production from the zone can be lost.
The migration of gas through the microannulus around the outside of the casing also increases the risk of well blowout. If the gas travels to a depleted zone, an underground blowout can occur. Alternatively, the newly pressurized zone can cause safety hazards for new drilling activity because the traditional warning signs of a high pressure zone, such as connection gas, background gas, temperature increases, changes in shale density, and penetration rate increases, may not be present in the artifically created pressure zone. If enough gas migrates through the microannulus to the well surface, the gas can escape in an uncontrolled fashion and cause a disastrous surface blowout.
Accordingly, a need exists for an improved apparatus and method for cementing wells that improves the bond between the cement and the exterior surface of the casing.